Microbial adaption to stoichiometric imbalances regulated the size of soil mineral-associated organic carbon pool under continuous organic amendments

Soil microorganisms play a key role in regulating soil organic carbon (SOC) accrual. Organic amendments with distinct stoichiometry may lead to imbalanced supply of carbon (C), nitrogen (N), and phosphorus (P) to the microbiome, causing changes in microbial community composition and their life strategies, as well as enzyme production. However, the response of soil microorganisms to these imbalances and whether their adaptive strategies are related to the fate of SOC pools remain largely unknown in low-fertility paddy soil. To address this uncertainty, soils were sampled from a 12-year experimental fertilisation trial under integrated application of mineral fertilizers and three types of organic materials (green manure, rice straw, and cattle manure). Stoichiometric imbalances between soil microbes and their available resources, enzyme activities, and microbial community composition, and their linkages with soil particulate (POC) and mineral-associated (MAOC) organic C were investigated. The results showed that despite equal C input, strongest increase in MAOC occurred with organic amendments causing the smallest microbial C:N imbalance and C:P imbalance, suggesting that alleviation in N and P limitation was inductive to the accrual of soil stable organic C fraction. Additional organic amendments with lower C: nutrient ratios shifted the microbial community towards the prevalence of r-strategists, with cattle manure addition supporting copiotrophic bacteria and green manure addition favouring copiotrophic fungi. Importantly, the relative abundances of Proteobacteria, Gemmatimonadetes, and Actinobacteria belonging to copiotrophs were negatively related to microbial C:N imbalance and C:P imbalance, but positively related to POC and MAOC, while Chloroflexi, Basidiomycota and Glomeromycota belonging to oligotrophs exhibited reversed relationships. In addition, greater MAOC accrual was associated with an increase in microbial biomass and a decrease in biomass-specific P-acquiring enzyme activity. Random forest analysis and partial least squares path model revealed that microbial C:N imbalance and C:P imbalance played an important but indirect role in shaping MAOC by concurrently regulating soil microbial biomass, community composition and enzyme production, whereas the POC pool was predominantly and directly controlled by the proportion of macroaggregates. These results provide empirical evidence for the stoichiometric control of microbial communities and their feedback to SOC pools, highlighting the role of low C:nutrient ratio organic amendments for long-term storage and the persistence of C in intensively managed paddy soils.